Energy converters utilising the Stirling Cycle (typically called “Stirling engines”) are well known and come in various configurations. A typical so called “alpha” type Stirling engine has two pistons reciprocating within respective cylinders. The cylinders are connected by a tube accommodating a special heat exchanger known as a regenerator. The pistons are both connected to a flywheel and a crankshaft. A working fluid of constant mass, typically gas, is hermetically contained within the cylinders and the tube. One cylinder, also known as a hot cylinder or an expansion cylinder, is connected with a heater to heat the fluid in that cylinder and the other cylinder, also known as a cold cylinder or compression cylinder is connected with a cooler to take heat away from that cylinder. The working fluid is cycled back and forth between the expansion cylinder and the compression cylinder, and passes through the regenerator twice in each cycle, while the regenerator alternately absorbs heat from, and releases heat to, the working fluid. The addition of heat to the expansion cylinder and the extraction of heat in the compression cylinder cause a series of compressions and expansions of the working fluid in the chambers, thereby causing the pistons in the chambers to reciprocate and to drive the crankshaft, which can provide work output in the form of rotational power. The regenerator retains a portion of the heat received in the expansion cylinder as the heated fluid passes from the expansion cylinder to the compression cylinder and gives the stored heat away as the fluid cooled in the compression cylinder flows in the opposite direction. The regenerator recycles heat which otherwise would be lost at the cold cylinder and thus increases thermal efficiency of a Stirling engine compared to other hot air engines. This part of the Stirling cycle is known as “regeneration”. Other types of a Stirling engine include so called “beta” and “gamma” types, which differ from the “alpha” type structurally, but operate under the same principle. A detailed discussion of the functioning of conventional Stirling engines is set forth in the book “Stirling Engines” by Graham Walker, Clarendon Press, 1980, the disclosure of which is incorporated herein by reference.
An attractive aspect of a Stirling engine nowadays is that it can be powered by practically any source of heat, including renewable energy sources such as sun and heat energy generated by wind. At the same time, a Stirling engine has several further advantageous features including that it produces virtually no atmospheric emissions and works with minimal noise.
Despite its apparent advantages, the efficiency of the Stirling engine is compromised by the series arrangement of the cooler, regenerator and heater. An ideal Stirling cycle assumes that the expansion and compression phases of the cycles occur isothermally. In reality this is unlikely to be the case since it is virtually impossible to provide a constant external inflow or outflow of heat conforming to the speed of piston strokes in order to maintain the same temperature during expansion or compression. Thus, it is often assumed that the expansion and compression phases occur adiabatically, i.e. the working fluid is heated upon compression and cooled upon expansion. A cycle occurring under this assumption is referred to as an ideal pseudo-Stirling cycle. In an ideal pseudo-Stirling cycle, after the fluid is heated by compression, it is cooled by the cooler at the compression cylinder, and then is heated again in the regenerator. Similarly the working fluid is re-heated by the heater after the working fluid has cooled during expansion in the expansion cylinder during a power stroke, and cooled again in the regenerator. This is counterproductive. An attempt to mitigate this drawback is disclosed in U.S. Pat. No. 2,724,248 to T. Finkelstein et al which describes a Stirling cycle machine incorporating one way “simple flap” valves.
A further attempt to mitigate this drawback is described in a thesis entitled “A Computer Simulation of Stirling Cycle Machines”, I. Urieli, Johannesburg, February 1977 submitted to the Faculty of Engineering University of the Witwatersrand, Johannesburg. This thesis publication describes one-way passive valves (and specifically discloses “simple flap valves”) respectively connecting each of the compression cylinder and the expansion cylinder directly with the regenerator to avoid unnecessarily cooling the compressed fluid and heating the expanded fluid on its way respectively from the compression and expansion cylinders to the regenerator. However, in this arrangement, when the bypass valve is open, the fluid can pass simultaneously through the bypass valve and the respective heater or cooler thus still resulting in a waste of energy.
The inventors have appreciated the need to further improve the efficiency of the above-described bypass arrangement.
Further attempts have been made to devise alternative machines to a Stirling cycle engine such as the alternative machine disclosed in US Patent publication number 2010/0186405 to Conde (assigned to Regen Power Systems LLS) and which discloses a closed cycle heat engine but which differs to a Stirling cycle machine because it has a working fluid flow path for the heated air which is separate from the working fluid flow path for the cooled air, the two working fluid flow paths remaining separated in a regenerator in the specific form of a twin path or counter-flow recuperator. Furthermore, US Patent publication number 2010/0186405 to Conde discloses an unbalanced system in which there are a greater number of expansion chambers than compression chambers which results in a greater working volume of the expansion chambers compared with the compression chamber.
Additionally, U.S. Pat. No. 5,720,172 discloses a flow controller for a Stirling cycle type engine, the flow controller having a pair of plate spring type valve plates which in practice act as simple one way valves and therefore provide a flow path controller, and a cock or throttle interposed in the said flow path which is used to control the flow rate along the flow path but only when fluid is able to flow along the flow path in the direction permitted by the plate spring type valve plate.
Accordingly, the object of the present invention is to provide a heat machine of a Stirling type having greater efficiency compared with prior art machines.